Many digital imaging systems enhance the contrast and lightness characteristics of digital images through the application of a tone scale curve. For a generalized tone scale curve T(x), the input pixel value x is transformed to an output pixel value T(x). The shape of the tone scale curve determines the visual effect imparted to the processed digital image. Some tone scale curves applied to digital image are independent of the pixel values in the digital image to be processed. Such image independent tone scale curves are useful for establishing a photographic look to the processed digital images. While image independent tone scale curves can be used to enhance many digital images, digital images that are either too high or low in contrast can benefit from the application of a tone scale curve that is responsive to the distribution of pixel values in the digital image to be processed. For image dependent tone scale curves, the mathematical formula used to generate the tone scale function determines the degree and nature of the image enhancement.
When a compressive tone scale function, i.e. the tone scale function designed to reduce the numerical range of pixel values, is applied directly to image pixel data, the spatial detail in the image can be diminished. In addition, if the compressive tone scale function is applied directly to a color digital image that is in a red-green-blue representation, the tone scale function can reduce the colorfulness (saturation) characteristics of the processed digital image. In general, better results can be achieved if the compressive tone scale function is applied directly to the luminance signal of a luminance-chrominance image representation. This procedure leaves the chrominance signals unchanged but can lead to processed images that have diminished spatial detail.
In the method disclosed by Gallagher and Gindele in U.S. Pat. No. 6,167,165, the spatial detail of the processed image can be mostly restored. In this method, a tone scale function is applied directly to the luminance signal and the spatial frequency detail of the tone scale modified luminance signal is adjusted inversely proportional to the slope of the applied tone scale function. The resultant processed digital images have more spatial detail and appear more natural.
A spatial filter can be used to apply a tone scale function to a digital image in such a manner that the image spatial detail will be relatively unaffected. This is particularly useful for compressive tone scale functions designed to reduce the image dynamic range. Such spatial filtering methods have been disclosed in U.S. Pat. Nos. 6,317,521 and 6,285,798. When applying compressive tone scale functions to the luminance signal with a spatial filter, the color characteristics of the processed digital images look more natural even though the chrominance signals are not modified.
In U.S. Pat. No. 6,317,521 Gallagher and Gindele disclose spatial filtering method for applying tone scale functions to digital images. In this method, the luminance channel of a luminance-chrominance representation digital image is separated into two signal parts. A tone scale function is applied directly to one of the signal parts and the other signal part is added back to the modified signal part. Tone scale functions generated with the methods disclosed in U.S. Pat. Nos. 4,731,671 and 4,745,465 have been applied with the spatial filtering method disclosed in U.S. Pat. No. 6,317,521 with success. In general, more spatial detail is preserved in the enhanced digital images than if a spatial filter were not used.
Low contrast images are enhanced by applying an expansive tone scale function designed to increase the numerical range of pixel values. Simple histogram stretching methods are well known in the art. Typically a histogram is calculated from the image pixel values and the minimum and maximum pixel values are obtained. A tone scale function is constructed using a linear transform equation that expands the pixel values to achieve a predetermined goal. For example the minimum pixel value is mapped to a lower pixel value while the maximum pixel value is mapped to a higher pixel value. Applying an expansive tone scale function to image pixel data when in a red-green-blue representation has the effect of increasing image contrast, color saturation, spatial detail, and noise. In general, the increase in contrast and spatial detail is viewed as an improvement in image quality. However, the increase in noise can reduce image quality.
In U.S. Pat. No. 6,285,798 Lee discloses a method for construction a compressive tone scale function and a wave-let based spatial filtering technique for applying the compressive tone scale function to image data. This spatial filtering technique helps preserve image detail while enhancing the tonal characteristics with the compressive nature of the tone scale function. That is, the tone scale functions produced by Lee's method compress, or reduce, the scene dynamic range of the image data. Lee's method does not account for the possibility that some digital images require an expansion of the dynamic range of the digital image to achieve enhancement.
The above mentioned methods are individually designed to either avoid or remedy specific problems with image spatial detail when for applying tone scale functions to digital images.